1. Field of the Invention
The present invention is directed to an apparatus and method for producing an extruded cellular wood-plastic composite. The present invention is specifically directed to a system for manufacturing extruded wood-plastic composite of low density comprised of soft wood or hard wood flour and a thermoplastic resin injected with a gas such as carbon dioxide (CO2) gas, nitrogen (N2) gas, or air while the material is preferably between dies and after the extruder. The resin may be polyethylene, polypropylene, or any other thermoplastic suitable for use in a wood plastic composite. The wood plastic composite produced by this process may be preferably in the form of a foam core having a skin around it.
The current state of the art in the extrusion of cellular wood plastic composites produces cellular wood plastic composites containing poly-vinyl chloride (PVC), acrylonitrile butadiene styrene (ABS), polypropylene (PP), or polystyrene (PS) as the plastic resin. Foaming is accomplished through the use of chemical blowing agents that add an appreciable amount to the cost of the product. Significant weight reduction of the extruded material must be accomplished by the use of these blowing agents to offset the increased cost. The foams so produced are usually allowed to expand outward upon exiting from the die. The extruded profile is then shaped and sized by methods known to those practiced in the art. In an alternative process, the composite may be extruded as a hollow profile and allowed to expand or foam inward. The profile is contained in an appropriate device to maintain the extruded shape in this alternate process.
2. Discussion of the Related Art
Historically, physical blowing agents like carbon dioxide have been successfully used to form low-density micro-cellular plastics in an extrusion process. However, the process required injection of the blowing agent at high pressures, utilized high-impact polystyrene (HIPS) as the plastic resin, and did not involve a wood-plastic composite material. Extrusion processes using physical blowing agents, such as carbon dioxide, currently produce cellular plastic material made from high-density polyethylene (HDPE) and PP. However, the gas is introduced in high-pressure regions of the extruder and must be pumped into the extrusion at pressures between 1000 and 5000 psi.
Strength and stiffness of wood plastic composites decrease when they are foamed. This decrease is generally inversely proportional to the density of the foam. As a result, currently available elements such as deck boards are either of relatively high density (0.8–0.9 g/cm2) or have a dense unfoamed, structural layer co-extruded on the outside of the foam. The co-extrusion process requires the use of a second extruder, which increases production times and costs.
What is needed therefore is an apparatus and process for better controlling the gas injected in the material to produce a more structurally sound plank. Further, what is also needed is an apparatus and process that is more efficient, practical and economical.